The invention relates to cellulose fibers or yarns with a reduced tendency to form fibrils and a process for manufacturing such fibers or yarns, whereby the fibers are preferably produced according to the NMMO filament formation process.
Cellulose fibers and yarns have long been known. The most important classic production processes are the so-called cuprammonium process and the viscose process.
It has also long been known how to dissolve cellulose polymers in an amine oxide of a tertiary amine, if necessary in the presence of water, and to produce from these solutions, by means of pressing through nozzle tools, formed objects such as fibers, filaments, yarns, films, and the like. Processes using N-methylmorpholine-N-oxide have turned out to be particularly suitable; economical interests and development efforts are centered on those processes. Processes for the production of such formed objects using N-methylmorpholine-N-oxide, in the following called NMMO processes, essentially consist in that, first, a suspension is produced from cellulose such as cotton linters, chemical wood pulp and the like, water and NMMO and in that this suspension is transformed into a solution by heating and removing a portion of the water.
This solution is then filtered and extruded through a nozzle into a mostly aqueous coagulation bath, preferably with an interim air gap, whereby the formed objects such as filaments, yarns, films and the like are formed via coagulation. These formed objects are then washed to remove any tertiary amine oxide still present. Subsequently the formed object can be dried and further processed in the customary manner, e.g. wound up, etc.
Compared to the classic processes for manufacturing cellulose formed objects, the NMMO process is in particular characterized in that it involves essentially physical phenomena, so that at least in theory no chemical reactions take place and no chemical byproducts are formed which must be disposed of as waste products or transformed back by chemical methods into the initial substances. The NMMO process therefore fundamentally ranks among the very environmentally friendly processes. Additionally the actual initial substance is a raw material which grows back, and the cellulose final product is highly biodegradable.
However, it has been shown that the cellulose fibers, especially those which are produced according to the NMMO process, exhibit a tendency to form fibrils, in particular in a wet state, especially if mechanical forces act on the fibers. This happens in the case of dyeing, among others, as well as during washing of the fibers, when after leaving the coagulation bath the solvent still present on the fibers is to be removed. Naturally, in all further processing steps the existing fibrils will be more or less conspicuous, in the dried state as well.
Dust is increasingly formed, and fine fibrils break off and roll together in curl fashion. Entire fibrils may even break off.
It may be true that the formation of fibrils can be useful in creating special surface effects, but for most applications fibrils are not desired.
Efforts have been undertaken to counteract the disadvantages of fibril formation in that e.g. dyed fabrics are treated with commercial cellulose cross-linking agents which have low formaldehyde content. By doing so the formation of fibrils in the fabric is reduced, although the rougher texture that the fabric exhibits must be tolerated.
Besides other disadvantages, a corresponding cross-linking prior to dyeing has the consequence that the dye receptivity is considerably reduced.
One further process for reducing the formation of fibrils is described in the international patent application WO 92-07124. This process consists essentially in that the cellulose fibers, which are not yet dried, are treated with an aqueous solution or dispersion of a polymer possessing a plurality of cationic groups. Since these polymers can be washed out very easily, it is recommended to also use a cross-linking agent, especially together with a catalyst. This process likewise diminishes the dye receptivity, and the elongation of the fibers is reduced.
Even though numerous methods are known to reduce the formation of fibrils in cellulose fibers, there is still a need for improved fibers and yarns with reduced fibril formation, as well as for improved and economically viable processes for manufacturing such fibers.
Therefore the objective of the present invention is to provide cellulose fibers and yarns, in particular such cellulose fibers and yarns which were obtained according to the NMMO process, which exhibit a reduced tendency to form fibrils, but which at the same time have a very good dye receptivity, i.e., a dye receptivity which essentially corresponds to that of untreated fibers or a dye receptivity which is only negligibly reduced, and whose mechanical textile properties, especially the elongation, are not or only negligibly affected compared to untreated fibers. A further objective of the invention is to provide a corresponding process by which such fibers are accessible, a process which operates economically, is conducive to reproducible results, operates continuously, allows a high spinning speed and does not require subsequent cleaning or neutralization steps in this connection.